This invention relates to a protected switch.
U.S. Pat. No. 4,893,158 describes a protected switch in the form of a power semiconductor device having first and second main electrodes for coupling a load between first and second voltage supply lines, a control electrode coupled to a control voltage supply line and a sense electrode for providing in operation of the device a current that flows between the first and sense electrodes and is indicative of the current that flows between the first and second main electrodes. A control semiconductor device has first and second main electrodes coupled between the control electrode and the second main electrode of the power semiconductor device and a control electrode coupled to the sense electrode of the power semiconductor. The control electrode of the control semiconductor device is coupled via a current path provided by a sense resistor to the second main electrode of the power semiconductor device so that, when the current provided by the sense electrode approaches the given current, the control semiconductor device is rendered conducting to provide a conductive path between the control and the second main electrodes of the power semiconductor device to regulate the operation of, in particular to regulate the current through, the power semiconductor device.
In the protected switch described in U.S. Pat. No. 4,893,158, the control semiconductor device effectively forms an inverter with the series gate resistance Ri of the power semiconductor device, the sensed current through the sense resistor Rs determining the gate drive. This results in a low gain which makes the regulation of the current of the power semiconductor device imprecise and dependent on the external gate drive, the value of the gate series resistance Ri and process parameters. Also, the negative temperature coefficient of the control semiconductor device at low threshold voltages, that is close to the point at which the control semiconductor device becomes conducting, and the typically positive temperature coefficient of the sense resistor result in an overall negative temperature coefficient for the output current limiting value of the circuit. In addition, where as described in U.S. Pat. No. 4,893,158, the sense electrode is provided by a current mirror MOS component made up of one or more cells substantially identical to the cells of the main power semiconductor device, the ratio between the current through the power semiconductor device and the current supplied through the relatively high value sense resistor Rs from the sense electrode will be a function of the process-dependent electrical parameters of the power semiconductor device and the current mirror or sense cell as well as their geometrical ratio because these two devices will be operating with dissimilar voltages between their control and second main electrodes and between their first and second main electrodes. This makes the operation of the design difficult to predict and results in poor precision. Furthermore, when the voltage between the first and second main electrodes of the power semiconductor device is low, the circuit may not function because the resultant voltage at the control electrode of the control semiconductor device may not be sufficient to turn on the control semiconductor device when required. As the voltage between the first and second main electrodes of the power semiconductor device rises, then the circuit will enter a negative resistance region as it begins to operate which may result in oscillatory action with certain types of loads such as inductors.